This invention relates generally to refrigeration systems and in particular, to a method for preventing overheating of the compressor of a refrigeration system, including those systems utilizing scroll type compressors. The invention is particularly well-suited for converting an existing refrigeration system using one refrigerant having particular physical and thermodynamic properties to use with another refrigerant having significantly different properties.
The design specifications of a refrigeration system are generally predicated on the choice of specific refrigerant to be utilized, i.e., on its physical and thermodynamic properties. For years, chlorofluorocarbons, e.g., CFC-12 or R-12; CFC-502 or R-502, had been used in compression refrigeration systems. These chlorofluorocarbons have excellent stability and were well suited for low temperature applications.
During the past two decades, it has been found that such chlorofluorocarbons released into the earth's atmosphere were depleting the ozone layer. Reduction in the ozone layer has been linked to many effects such as an increased risk for skin cancer. In response to concerns over ozone layer depletion, the U.S. government has imposed increasingly stricter limitations on the use of these refrigerants. These limitations require the phase out of the commonly used refrigerants with other refrigerants considered not so effect the ozone layer.
Currently, many commercial refrigeration systems utilize R-502 and the design features of such systems are dictated by the properties of R-502, e.g., type, size and operating parameters of the compressor. The phase out of R-502 in favor of other refrigerants, such as R-22 or AZ-50, is not a simple matter of removing the refrigerant from the existing system and replacing it with the environmentally preferred refrigerant. The physical and thermodynamic properties of, e.g., R-22, refrigerant are significantly different from those of R-502 such that the refrigeration system operates with different performance parameters than those required by R-502.
In the normal compression refrigeration cycle, vapor refrigerant is drawn into a compressor where it is compressed to a higher pressure. The compressed vapor refrigerant is cooled and condensed in a condenser into a high pressure liquid which is then expanded, typically through an expansion valve, to a lower pressure and caused to evaporate in an evaporator to thereby draw heat and thus, provide the desired cooling effect. The expanded, relatively low pressure vapor refrigerant exiting the evaporator is once again drawn into the compressor and the cycle starts anew.
A variety of compressor types have been used in refrigeration systems, including reciprocating compressors, screw compressors, rotary compressors and scroll compressors. Scroll compressors are becoming increasingly popular due to their capability for extremely high operating efficiency as compared to reciprocating, screw and rotary compressors. Scroll compressors are constructed using two scroll members with each scroll member having an end plate and a spiral wrap. The spiral wraps are arranged in an opposing manner with the two spiral wraps being interfitted. The scroll members are mounted so that they may engage in relative orbiting motion with respect to each other. During this orbiting movement, the spiral wraps define a successive series of enclosed spaces, each of which progressively decreases in size as it moves inwardly from a radially outer position at a relatively low suction pressure to a central position at a relatively high pressure. The compressed gas exits from the enclosed space at the central position through a discharge passage formed through the end plate of one of the scroll members.
A problem that all compressors, including scroll compressors, have in common is the need to avoid excessive heating of the compressor during high load operation. The action of compressing the vapor refrigerant imparts work onto the vapor and results in a significant increase in the vapor temperature. While a substantial portion of this heat is subsequently transferred to the atmosphere during the condensation process, a portion of the heat is transferred to the compressor components. Depending upon the specific refrigerant vapor compressed and on the pressure conditions operation, this heat transfer can cause the temperature of the compressor components to overheat, resulting in degradation of compressor performance, of the compressor lubricant or oil, and potentially damage to the compressor itself. For example, it has been found that the direct substitution of R-22 for R-502 in an existing refrigeration system results in high discharge temperatures, particularly under high load situations and high compression ratios.
One solution for converting existing systems using R-502 to R-22 or other substitutes calls for the replacement of expensive equipment, e.g., the compressor or supplementation of the existing condenser, resulting in significant capital costs as well as higher operating costs due to increase capacity needed for the compressor and condenser. Some prior art systems have attempted to respond to this problem. See, e.g., U.S. Pat. No. 5,189,883 issued to Bradford which discloses a refrigeration retrofit system utilizing a liquid refrigerant injection system, and U.S. Pat. No. 5,640,854 issued to Fogt et al., U.S. Pat. No. 5,329,788 issued to Caillat et al., U.S. Pat. No. 5,076,067 issued to Prenger et al. and U.S. Pat. No. 4,974,427 issued to Diab which also disclose a liquid refrigerant injection system for limiting or controlling excessive discharge gas temperature. These prior art systems, however, require the installation of multiple components to an existing system, require significant structural modification to an existing system or do not permit at all modification to an existing system.
Despite recognition and study of various aspects of the replacement refrigerant problem, the prior art has still not produced a simple, economical way to prevent overheating especially in converting existing refrigeration and air conditioning systems designed, e.g., for R-502, to the use of newer, environmentally preferred refrigerants.